Polycarbonate has been used extensively to mold medical devices such as blood filters, blood oxygenators and the like. Prior to use, these medical devices must be sterilized, preferably by exposure to ionization radiation, e.g. gamma radiation or electron beam radiation, for a prescribed period of time. While the physical and mechanical properties of polycarbonate are substantially unaffected by this radiation, the exposure results in an often undesirable change in the color, a general yellowing, of the resin.
It has been the object of several researchers in the field to identify the means to reduce this objectionable formation of color. The patent literature is noted to include U.S. Pat. Nos. 4,804,692; 4,874,802 and 4,904,710 which disclosed gamma radiation resistant polycarbonate compositions containing polyether polyols.
Also relevant is 11.08.88-JP198940 for the disclosure that a polycarbonate composition containing an ester having mercapto group of thio glycolic acid or mercapto-propionic acid and alcohol is capable of being sterilized by exposure to radiation. A radiation sterilizable composition used for medical devices containing a semi-crystalline polymer, a hindered phenolic stabilizer, phosphite and a thioester (or thioether) has been disclosed in EP 303,894. Enhancing the color stability to sterilization radiation of polymer compositions has been disclosed in U.S. Pat. No. 4,939,185. Accordingly, compounds having defined thioether groups, including dithiane, are said to stabilize aromatic polycarbonate upon exposure to sterilizing radiation. Compounds having sulfone functional groups and thiazole functional groups have been disclosed to enhance the color stability of polycarbonate compositions in U.S. Pat. No. 4,880,855 and 4,880,856 respectively.
U.S. Pat. Nos. 4,067,846 and 3,909,490 are noted to disclose flame retardant polycarbonate compositions containing metal salts of sulfonic acids of aromatic sulfides. A cyclic polycarbonate oligomer is disclosed in U.S. Pat. No. 4,751,283. The oligomer may be prepared from monomers containing aromatic disulfides.